First step: make sure to have a plan B ready, before making any other steps in the negotiation process. Look for jobs similar to yours and that pay the same or better. From there, the situation becomes rather simple. If you walk away, the product will become abandonware and will take a big hit in value, say A dollars. Given plan B, walking away would cost you nothing at all. The maximum you can get from the new owners for staying on, is A dollars. The fair amount would be A/2 dollars. If you do not get anything at all, execute plan B and walk away.
